1. Field of the Invention
This invention relates to a reader-printer changeable over between a reader mode in which the light image of an original is projected onto a screen for observation and a printer mode in which the light image of the original is projected onto a photosensitive medium by a slit exposure system and the copy image thereof is obtained.
2. Description of the Prior Art
In the reader-printer of this type, it has heretofore been practised that the reader optical path for projecting the image of an original such as a microfilm onto a screen and the printer optical path for projecting the image of the original onto a photosensitive medium are used partly in common with each other. To change over the apparatus between the reader mode and the printer mode, a change-over mirror is provided in the common optical path and by rotating this mirror, it is placed into and out of the optical path to thereby effect the change-over between the two modes. This change-over method has required a driving device for rotating the change-over mirror, and this has led to the complexity of the construction of the apparatus as well as the bulkiness of the change-over mirror which is attributable to the necessity of reflecting the entire image and thus, it has been difficult to make the apparatus compact. Further, it is difficult to accurately dispose the rotatable mirror at a predetermined angle in the common optical path and, even if such mirror is accurately disposed, frequent change-over operation may derange the disposition angle of the mirror with a result that the image cannot be accurately projected onto a predetermined position.
Also, in a reader-printer wherein the copying operation is effected by a slit exposure system, it is necessary to move a scanning member to scan the original and this leads to the necessity of providing a second driving device for moving the scanning member and thus, the conventional reader-printer has been provided with the device for driving the change-over mirror and the device for driving the scanning member. This has led to the complexity, bulkiness and expensiveness of the apparatus.
Generally, originals to be copied in a copying apparatus include those of high contrast such as printed matter and those of low contrast such as newspaper or diazo copies, and to obtain good copies, it is necessary to change the exposure amount by the exposure lamp or the bias voltage of the developing device. In such case, the density of the original is detected and the exposure amount, etc. are controlled in accordance with the detected density. The detection of the image density of the original is accomplished by detecting the light reflected by or transmitted through the whole or part of the original, and in such photometric system, when for example a document is to be copied, even if the characters on the document are of the same density, the photometric value (the quantity of light received) may differ depending on the portion which the characters occupy in the photometrically measured area (which corresponds to the light-receiving area of the light-receiving element) and thus, the quality of the detected copy image varies in conformity with the size of the photometrically measured area. To accurately detect the image density, photometry can be effected by a light-receiving element of a small light-receiving area, but in this case, it is necessary to provide a number of light-receiving elements to reliably detect the ground density of the original and the density of the images such as characters, and the increased number of light-receiving elements leads to an increased number of attendant circuits (for example, an output amplifying circuit, a photometric value detecting circuit, etc.) and accordingly complexity and expensiveness of the apparatus.